This invention relates to a gate valve with improved seals to the annular seat elements.
Gate valves of slab gate valve type typically include floating annular seat elements sealed in counterbores or seat pockets in the flowbore of the valve body on either side of the slab gate. The annular seat elements are mounted for “floating” or limited axial movement toward and away from the slab gate. Gate valves of this nature, the problems inherent with fines entering the sealing areas, and the prior art efforts to solve these problems are well reviewed in prior art patents, see for example U.S. Pat. Nos. 4,645,179; 5,029,812; 5,727,775; 6,279,875; 6,664,572; and 7,004,452. Gate valves of the expanding gate valve type are distinctly different from slab gate valves in that they typically include an expanding or split gate and fixed valve seats which have an interference fit relative with the seat pockets. The expanding gate is typically formed from wedge shaped gate members which slide against each other to “expand” the width of the gate as the gate is opened and closed. Alternatively, the gate might be split to accommodate springs. This expanding action seals the gate against the fixed seats. Fixed seats of this type suffer problems in that, despite the interference fit with their seat pocket, they are not positively held in place and can work their way out of the seat pockets, especially after the valve has been cycled repeatedly. Many locking and sealing mechanisms have been proposed to solve the problems of fixed seats of expanding gate valves. However, these problems are generally not faced by gate valves with floating seats, where the annular seat elements are designed to move (float) upstream and/or downstream with the opening and closing of the slab gate member, such that the opposing seal surfaces (or faces) of the annular seat elements seal to the gate on one seal surface and to the seat pocket on the opposing seal surface. In the slab gate valve designs, carrier rings having an interference fit to the seat pockets might be used, in which case the annular seat elements seal against the gate on one side, and the carrier ring on the other side. Gate valves are also known which have expanding or split gate, but which also use floating annular seat elements, although these are less common.
In the slab gate valve design with floating seat elements, tight clearances and metal-to-metal sealing surfaces are maintained between the valve body (i.e., seat pockets with optional metal carrier rings), the annular seat element and the gate. Despite the tight clearances, gaps exist between all surfaces. The upstream annular seat element (generally at the inlet) and the gate float downstream (generally toward the outlet) with the pressure to seal against the downstream annular seat element. Particles smaller than the gap between the sealing surfaces may enter the gap. Flow through a gate valve is usually in one direction, with the gate valve holding pressure in the opposite direction. With a slab gate valve type having floating annular seat elements, the flow and sealing can occur from either direction, to be a bidirectional valve. Over time, fine particles can get in between all surfaces, preventing the metal-to-metal seal from forming, and eventually produce leakage.